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Thread: A380 Rudder Reversals

  1. #21
    Senior Member Gabriel's Avatar
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    I have things to say here, but I don't have time now.
    But I will say this. The video is long with several differently edited takes of the same approach and landig, you have close ups, slomo, longer takes..
    Watch it ALL and, above all, appreciate the vegetation. Seriously, that's a cue.

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  2. #22
    Junior Member Roshni's Avatar
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    I liked this crosswind video. Being a newbie I am knowing and getting familiar with different things.

  3. #23
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    Quote Originally Posted by Evan View Post
    A reversal, which we are witnessing here, is a successive cyclic rudder deflection from one direction to the opposite direction without first centering it in the neutral position. It doesn't appear to be full rudder deflection reversals but who knows. The structural danger in doing this probably isn't present at landing speeds, but overcontrol can result and this can lead to further occillations, as I believe we seeing here.

    It's a FBW Airbus, so the pilots can do whatever they want and it's still safe.

  4. #24
    Senior Member Gabriel's Avatar
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    Cue 2 and 3...

    Why does it require to have a sing down and the ball off-center to keep coordinated flight with an engine our?
    May the force be with you.

    (cue 1 was "appreciate the vegetation")

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  5. #25
    Senior Member Gabriel's Avatar
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    Ok, here is my analysis, for whatever is worth that I know some think is nothing if not less than that.

    Disclaimer, I have not seen any METARs or weather info, and my analysis is bases solely in what's seen in the video + laws of physics.

    The plane was approaching with a 1) fair crosswind from the right (as seen from the pilot's seat, which is from the left of the video frame).
    The wind was obviously not steady, there was some 2) turbulence, 3) some windshare (significant change in wind intensity and/or direction that takes place in a small vertical or horizontal distance), and 4) wind gradient (gradual change in wind speed with altitude).

    The 1 is obvious from the crab to the right (ok, I mean a crab with the nose to the right compared to the flight path).

    The first 2 are obvious for the video take that captures a larger part of the approach. In that video it looks like the turbulence is very strong and sudden, but keep in mind that this take is played at higher than normal speed.

    The last is obvious from the vegetation. The leaves of the trees barely move, indicating a low surface wind.

    The pilot had to fight with all that, and they didn't do it great.

    Shortly (but not immediately) before landing, the plane starts to deviate to the right even when the nose was still pointing in the same general direction. This is probably due to a reduction in the crosswind speed.

    Then, immediately before and during touchdown, the pilot applies what seems to be a full travel and somehow long left rudder input. This has 2 effects: of course the nose goes to the right. But why does the nose that? Because the left rudder generates a left yawing moment. Why? Because the left rudder generates a right lift on the fin which, combined with the long arm of the fin behind the CG, generates a left yawing moment.

    But what happens with the right force to begin with? Yes, moment, yaw, yadayada. But what about F=ma?
    Exactly. Big left rudder deflection = big lift on the fin towards the right = big acceleration towards the right.

    The plane, which was already starting to move the the right due to the reduction of the crosswind, suddenly has a significant force to the right. So right we go.

    The pilot keeps the large left rudder input through the touchdown, and not only as needed to align the nose with the runway, the pilot keeps it through the nose passing from aiming left to aiming right. The pilot was trying to steer the plane with yaw, something that is not effective until the wheels are on the ground, the spoilers fully extended, and the noseweel down on the ground. That's why most of the acceleration to the right happens with the main wheels already on the ground, despite the nose of the plane moving left and against the main wheels skidding to the right.

    The spoilers extend, the nose go down, and now we have a significant weight on the main wheels and hence a significant traction and, with the nose pointing left, left we go.

    At that point we enter a PIC or PAC (pilot-induced oscillation or pilot-airplane coupling). The pilot wants to steer for yaw, instead of steering for yaw rate (which is what you need to avoid or kill an oscillation). So, for example, the pilot reverse from rudder significantly left to rudder significantly right when the yaw goes through what the pilot feels is the right heading to go back to the centerline, but then the plane keeps yawing left a little more even with right rudder to inertia. Then the left yawing stops and, still with right rudder applied, it starts yawing right and passes through a point where it is aligned with the runway heading and more or less on the centerline. At that point the pilot reverses again to significant left rudder, but the plane keeps yawing right a bit more due to inertia.

    The oscillation is however damped and after a few come and go the plane ends up on a solid and stable ground track.

    The oscillation could have been killed much earlier had the pilot applied reverse rudder before reaching the target yaw, so as the yaw rate would slow down and stop by the target (or close), then center the rudder at zero yaw rate as to not induce any new yaw rate, and then fine tune it as needed.

    Overall, it was a good landing by definition, but not a nice one, the pilot did a fair job in challenging conditions, but could have done better.

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  6. #26
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    Quote Originally Posted by Gabriel View Post
    Ok, here is my analysis, for whatever is worth that I know some think is nothing if not less than that.

    Disclaimer, I have not seen any METARs or weather info, and my analysis is bases solely in what's seen in the video + laws of physics.

    The plane was approaching with a 1) fair crosswind from the right (as seen from the pilot's seat, which is from the left of the video frame).
    The wind was obviously not steady, there was some 2) turbulence, 3) some windshare (significant change in wind intensity and/or direction that takes place in a small vertical or horizontal distance), and 4) wind gradient (gradual change in wind speed with altitude).

    The 1 is obvious from the crab to the right (ok, I mean a crab with the nose to the right compared to the flight path).

    The first 2 are obvious for the video take that captures a larger part of the approach. In that video it looks like the turbulence is very strong and sudden, but keep in mind that this take is played at higher than normal speed.

    The last is obvious from the vegetation. The leaves of the trees barely move, indicating a low surface wind.

    The pilot had to fight with all that, and they didn't do it great.

    Shortly (but not immediately) before landing, the plane starts to deviate to the right even when the nose was still pointing in the same general direction. This is probably due to a reduction in the crosswind speed.

    Then, immediately before and during touchdown, the pilot applies what seems to be a full travel and somehow long left rudder input. This has 2 effects: of course the nose goes to the right. But why does the nose that? Because the left rudder generates a left yawing moment. Why? Because the left rudder generates a right lift on the fin which, combined with the long arm of the fin behind the CG, generates a left yawing moment.

    But what happens with the right force to begin with? Yes, moment, yaw, yadayada. But what about F=ma?
    Exactly. Big left rudder deflection = big lift on the fin towards the right = big acceleration towards the right.

    The plane, which was already starting to move the the right due to the reduction of the crosswind, suddenly has a significant force to the right. So right we go.

    The pilot keeps the large left rudder input through the touchdown, and not only as needed to align the nose with the runway, the pilot keeps it through the nose passing from aiming left to aiming right. The pilot was trying to steer the plane with yaw, something that is not effective until the wheels are on the ground, the spoilers fully extended, and the noseweel down on the ground. That's why most of the acceleration to the right happens with the main wheels already on the ground, despite the nose of the plane moving left and against the main wheels skidding to the right.

    The spoilers extend, the nose go down, and now we have a significant weight on the main wheels and hence a significant traction and, with the nose pointing left, left we go.

    At that point we enter a PIC or PAC (pilot-induced oscillation or pilot-airplane coupling). The pilot wants to steer for yaw, instead of steering for yaw rate (which is what you need to avoid or kill an oscillation). So, for example, the pilot reverse from rudder significantly left to rudder significantly right when the yaw goes through what the pilot feels is the right heading to go back to the centerline, but then the plane keeps yawing left a little more even with right rudder to inertia. Then the left yawing stops and, still with right rudder applied, it starts yawing right and passes through a point where it is aligned with the runway heading and more or less on the centerline. At that point the pilot reverses again to significant left rudder, but the plane keeps yawing right a bit more due to inertia.

    The oscillation is however damped and after a few come and go the plane ends up on a solid and stable ground track.

    The oscillation could have been killed much more earlier had the pilot applied reverse rudder before reaching the target yaw, so as the yaw rate would slow down and stop by the target (or close), then center the rudder at zero yaw rate as to not induce any new yaw rate, and then fine tune it as needed.

    Overall, it was a good landing by definition, but not a nice one, the pilot did a fair job in challenging conditions, but could have done better.
    Once again, ^^^ 100% ^^^

  7. #27
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Ok, here is my analysis, for whatever is worth that I know some think is nothing if not less than that.

    Disclaimer, I have not seen any METARs or weather info, and my analysis is bases solely in what's seen in the video + laws of physics.

    The plane was approaching with a 1) fair crosswind from the right (as seen from the pilot's seat, which is from the left of the video frame).
    The wind was obviously not steady, there was some 2) turbulence, 3) some windshare (significant change in wind intensity and/or direction that takes place in a small vertical or horizontal distance), and 4) wind gradient (gradual change in wind speed with altitude).

    The 1 is obvious from the crab to the right (ok, I mean a crab with the nose to the right compared to the flight path).

    The first 2 are obvious for the video take that captures a larger part of the approach. In that video it looks like the turbulence is very strong and sudden, but keep in mind that this take is played at higher than normal speed.

    The last is obvious from the vegetation. The leaves of the trees barely move, indicating a low surface wind.

    The pilot had to fight with all that, and they didn't do it great.

    Shortly (but not immediately) before landing, the plane starts to deviate to the right even when the nose was still pointing in the same general direction. This is probably due to a reduction in the crosswind speed.

    Then, immediately before and during touchdown, the pilot applies what seems to be a full travel and somehow long left rudder input. This has 2 effects: of course the nose goes to the right. But why does the nose that? Because the left rudder generates a left yawing moment. Why? Because the left rudder generates a right lift on the fin which, combined with the long arm of the fin behind the CG, generates a left yawing moment.

    But what happens with the right force to begin with? Yes, moment, yaw, yadayada. But what about F=ma?
    Exactly. Big left rudder deflection = big lift on the fin towards the right = big acceleration towards the right.

    The plane, which was already starting to move the the right due to the reduction of the crosswind, suddenly has a significant force to the right. So right we go.

    The pilot keeps the large left rudder input through the touchdown, and not only as needed to align the nose with the runway, the pilot keeps it through the nose passing from aiming left to aiming right. The pilot was trying to steer the plane with yaw, something that is not effective until the wheels are on the ground, the spoilers fully extended, and the noseweel down on the ground. That's why most of the acceleration to the right happens with the main wheels already on the ground, despite the nose of the plane moving left and against the main wheels skidding to the right.

    The spoilers extend, the nose go down, and now we have a significant weight on the main wheels and hence a significant traction and, with the nose pointing left, left we go.

    At that point we enter a PIC or PAC (pilot-induced oscillation or pilot-airplane coupling). The pilot wants to steer for yaw, instead of steering for yaw rate (which is what you need to avoid or kill an oscillation). So, for example, the pilot reverse from rudder significantly left to rudder significantly right when the yaw goes through what the pilot feels is the right heading to go back to the centerline, but then the plane keeps yawing left a little more even with right rudder to inertia. Then the left yawing stops and, still with right rudder applied, it starts yawing right and passes through a point where it is aligned with the runway heading and more or less on the centerline. At that point the pilot reverses again to significant left rudder, but the plane keeps yawing right a bit more due to inertia.

    The oscillation is however damped and after a few come and go the plane ends up on a solid and stable ground track.

    The oscillation could have been killed much earlier had the pilot applied reverse rudder before reaching the target yaw, so as the yaw rate would slow down and stop by the target (or close), then center the rudder at zero yaw rate as to not induce any new yaw rate, and then fine tune it as needed.

    Overall, it was a good landing by definition, but not a nice one, the pilot did a fair job in challenging conditions, but could have done better.
    WIth a fin of that magnitude, do you really need reversals to get things centered? I would expect the plane to recover from the gust without much, if any, opposite rudder correction, and all that would be necessary is what is needed to decrab before touching down. These were very extreme, gusting conditions, so a steady-state crosswind input is going to experience some upset but is it really better to use rudder to recover or to simply let the plane recover on its own? I've read lit from Boeing, Airbus and the FAA emphasizing that rudder controls on a large transport aircraft are only intended for steady-sideslip and engine failure. The Boeing 'maxim' was "If you are in a stall, don't use rudder. If you are not in a stall, you don't need rudder" (obviously that doesn't include very high AoA upsets where rudder is all you have left). But the operative word there is "need", as the force of the fin itself will facilitate recovery from yaw excursions.

    The reason I posted this is to question whether some pilots are STILL not being trained properly for rudder on these big jets and are STILL using them with a small plane mentality.

  8. #28
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    WIth a fin of that magnitude...,
    The fin of that magnitude, the yaw inertia is of that magnitude.

    do you really need reversals to get things centered?
    No, as I clearly explained in my previous post. Neither with a fin of that magnitude or with a C-152.
    The problem is when you hold the input too much, then you may need some reversal. Actually, rudder reversal is the normal input done by the yaw damper.
    input = -k1*yaw - k2*(yaw rate) -k3*(yaw integral in the last t seconds).
    What is called a PID control.

    I would expect the plane to recover from the gust ....
    What gust? I don't think that the yaw upset was caused by a gust. Rather, by... ok, read my previous post.

    By the way, there was not only a yaw upset, but also a lateral lineal motion upset. To a point, the pilot had to do something to keep the plane from going out to the right of the runway. Hadn't the pilot apply rudder, we would be reading an AvHerlad article of the type "temporary departure from the runway while landing". I don't know of many other options other than allpying left rudder ehrn he did immediately before the landing (a bit of left bank maybe, but I don't think it would have been enough). Now, he held it too long, and that required the reversal and started the PIO.

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  9. #29
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    The fin of that magnitude, the yaw inertia is of that magnitude.


    No, as I clearly explained in my previous post. Neither with a fin of that magnitude or with a C-152.
    The problem is when you hold the input too much, then you may need some reversal. Actually, rudder reversal is the normal input done by the yaw damper.
    input = -k1*yaw - k2*(yaw rate) -k3*(yaw integral in the last t seconds).
    What is called a PID control.


    What gust? I don't think that the yaw upset was caused by a gust. Rather, by... ok, read my previous post.
    There was a very powerful storm system moving through northern Germany and large, sudden gusting winds around that time. I assumed that was the culprit.

    I don't know of many other options other than allpying left rudder ehrn he did immediately before the landing (a bit of left bank maybe, but I don't think it would have been enough).
    I would expect left bank and removal of the left rudder input to suffice. In all the A380/747 crosswind landings I've viewed, I've never seen a significant rudder reversal involved.

  10. #30
    Senior Member Gabriel's Avatar
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    At what point? How much would you bank 1 or 2 seconds before touchdown?

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  11. #31
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    At what point? How much would you bank 1 or 2 seconds before touchdown?
    Just til I head the scrapy sound.

    Com'on Gabriel, you've never seen bank used in a crosswing landing?

  12. #32
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Just til I head the scrapy sound.

    Com'on Gabriel, you've never seen bank used in a crosswing landing?
    Evan, you are not reading my posts. How much bank would you apply 2 seconds before touchdown? I am sure that between zero as you pretend that I am implying and the scrapy sound there is some range. How much would the plane even bank if you start banking 2 seconds before touchdown? How much would the airplane turn left at that bank and in that time? (or will you keep the plane banked after touchdown?). The nose was aiming right, The plane was to the right of the threshold and moving more and more to the right. What do you do? Do you really believe that 2 seconds of bank would have been enough?

    Anyway, I would say that they should have banked a bit. Hell, I would not say. I DID say it a few posts ago, those that you are not reading, but I very much doubt that that alone would have kept the plane from going off the side of the runway. For me the not-so-good piloting comes AFTER the touchdown, when the pilot enters that series of pilot-induced oscillations with rudder reversals. Up to after the touchdown, there was no clear or significant rudder reversal.

    Quote Originally Posted by Evan View Post
    Com'on Gabriel, you've never seen bank used in a crosswing landing?
    Yes, and typically into the wind, which in this case would be to the right.

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  13. #33
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    Quote Originally Posted by Gabriel View Post
    Evan, you are not reading my posts. How much bank would you apply 2 seconds before touchdown? I am sure that between zero as you pretend that I am implying and the scrapy sound there is some range. How much would the plane even bank if you start banking 2 seconds before touchdown? How much would the airplane turn left at that bank and in that time? (or will you keep the plane banked after touchdown?). The nose was aiming right, The plane was to the right of the threshold and moving more and more to the right. What do you do? Do you really believe that 2 seconds of bank would have been enough?

    Anyway, I would say that they should have banked a bit. Hell, I would not say. I DID say it a few posts ago, those that you are not reading, but I very much doubt that that alone would have kept the plane from going off the side of the runway. For me the not-so-good piloting comes AFTER the touchdown, when the pilot enters that series of pilot-induced oscillations with rudder reversals. Up to after the touchdown, there was no clear or significant rudder reversal.


    Yes, and typically into the wind, which in this case would be to the right.
    The thing is, I'm not a pilot and I never pretended to be one so I'm not going to get into an argument over the AMOUNT of control input that is needed. I am concerned here with the NATURE of the control input. I concentrate on what is learned in accident reports and what needs to be altered or improved on to make flying safer. What I see on this video appears to be a pilot using rudder too actively, applying too much rudder, for too long, in opposite directions and it results to an upset situation. That leads me to think that this pilot has insufficient ON TYPE motor skills training on rudder and/or has incorrect technique training on the use of rudder ON TYPE and/or was caught in a moment of aircraft-pilot coupling. I think it really deserves attention and hopefully the airline noticed this as well.

    The pilot who broke AA587 and killed all those on board had a history of aggressive rudder use. The report documents two occasions where the pilot was counseled by his flight captain for this behavior, The trouble is, nobody took it very seriously, nobody saw where it could lead to. Since that report came out, pilots have been given greater awareness of the role and use of rudder on large aircraft and the potential dangers and training programs have been modified to distinguish recovery techniques by type of aircraft. That's great. But how many pilots are still not getting this training? All it takes is one and some uncommonly bad luck.

    What do you do?
    In this case, it appears to me that the pilot is using rudder very actively as a primary control surface, the way you would in a much different, much smaller aircraft, and I don't think it was necessary. And if he was in danger of departing too far from the centerline with only some bank and more-or-less neutral rudder—without cranking over that much rudder for that long to align (and subsequently touch down with that much yaw rate), then I think that is reason to arrest it and go around.

  14. #34
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    Quote Originally Posted by Evan View Post
    WIth a fin of that magnitude, do you really need reversals to get things centered?
    Get over your reversal obsession. A little back and forth is normal (and ok, sometimes it looks impressive and full ish). 'A fin of that magnitude' needs the same relative swings as any other plane... the fin is big because the plane is big. The big fin dosn't make it more controllable, the big fin assures that it's properly controllable. Again... watch some other crosswind YouTubes, note the rudder deflection which is often visible. The rudder goes back and forth, as necessary, to address yaw. Get out of the bubble and drive your car in a crosswind. Note that you will move the steering wheel back and forth as-necessary.

    Plus, it's kind of hard to bust off a tailfin when a plane is full flaps slow. You really have to hit it just right and a bit faster.
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  15. #35
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    Quote Originally Posted by Evan View Post
    1. I am concerned here with the NATURE of the control input. I concentrate on what is learned in accident reports and what needs to be altered or improved on to make flying safer.

    2. In this case, it appears to me that the pilot is using rudder very actively as a primary control surface, the way you would in a much different, much smaller aircraft, and I don't think it was necessary.
    1. Sorry man... the deal is that you do those things with an exceedingly strong, black and white mentality... yes, really...and again, this one is on you, and not an excuse to play with my voodoo doll.

    2.

    A. The rudder is ALWAYS a primarily control surface. (say that 1000 times) Its nice that the yaw damper works it most of the time, but pilots are expected to make inputs during crosswind landings and when in the runway.

    B. Reread your own wing scrape comment-a beautiful combination of sarcasm and accuracy- that means you don't have much bank availability and must instead depend on rudder...

    C1. Down low..,
    And
    C2. In a crosswind...

    The rudder IS your primary directional control (see footnote).

    Does he use it too much in your opinion...(Read Gabriel's comment about running off the side of the runway). It's the black and white tone...he used the rudder too much, he's an idiot, training is all wrong, what I would have done...yeah, you don't say exactly that, but the tone is pretty clear.

    Was it less than genius rudder use?...perhaps so...but YouTube is full of not-so-pretty crosswind landings, and Gabriel and I and Bobby and ATL will testify that it is something of an art a skill to make the perfect inputs as you force the plane to fly in an uncoordinated manner to land on a somewhat narrow strip of concrete and with changing winds...there will be imperfections.

    Footnote: Ironingly, a high wing Cessna allows liberal use of bank for directional control... so the 'big planes are different from little ones' argument you always make... yeah, you need MORE rudder to steer the big one in a crosswind.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  16. #36
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    Quote Originally Posted by Gabriel View Post
    ***Then, immediately before and during touchdown, the pilot applies what seems to be a full travel and somehow long left rudder input.***
    When I look at this, I also note that the plane is somewhat to the right of centerline, and maybe a bit slow to start returning to the centerline.

    My $0.02 is that this input is largely appropriate.

    That being said, as the nose arrives at the centerline, the rudder still looks full left...so I concur that:

    1. He/She left it in just a little bit too long.

    and

    2. Yeah, ideally (and based on my position in a chair in front of a computer) he/she should have eased up (or maybe even neutralized) the thing as he/she approached the centerline.

    I continue to struggle with the fact that I feel some left and right oscillations on most airliner takeoffs (as well as remembering doing them myself) [and factor in the telephoto effect] Technically, it's PIO/sort-of-coupling...and no different than driving a car...but a very normal level of "oscillation" that does not amplify...and not meaning to dismiss the swerving as much as saying, it might not be as bad as it looked.
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  17. #37
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    Quote Originally Posted by 3WE View Post
    1. Sorry man... black and white mentality... voodoo doll.
    Now open your mind. Take a look at this one (or any of the hundreds of others). Notice the use of rudder. Notice the use of bank. You see rudder INPUT (not occillation) in one direction, then neutral rudder, then a modest amount in the opposite direction needed to decrab ALONG WITH BANK, DOWN LOW and a touchdown IN A BANK in what is obviously a very significant crosswind. There are no reversals involved. And it all ends with the aircraft stable on the centerline, not zigzagging down the runway. Watch as many of thee as you want, watch the rudder and watch the bank angle.

    https://www.youtube.com/watch?v=nehZ9v4YB6o

  18. #38
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    Quote Originally Posted by Evan View Post
    Just til I head the scrapy sound.

    Com'on Gabriel, you've never seen bank used in a crosswing landing?
    I am amazed at how often 'the slight bank' goes 'the wrong direction'.

    I have asked myself: "What are these stupid pilots being taught."

    BUT

    After further thought, I have reached a differing conclusion: You really have very limited bank freedom in a low-wing airliner...these 'stupid pilots' are aiming for 'as level as they reasonably can' and when the bank the wrong direction...it's a momentary imperfection caused by wind gusts, dihedral effect from crab adjustment, any number of things.

    I genuinely doubt that they intentionally bank away from the wind.
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  19. #39
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Now open your mind. Take a look at this one (or any of the hundreds of others). Notice the use of rudder. Notice the use of bank. You see rudder INPUT (not occillation) in one direction, then neutral rudder, then a modest amount in the opposite direction needed to decrab ALONG WITH BANK, DOWN LOW and a touchdown IN A BANK in what is obviously a very significant crosswind. There are no reversals involved. And it all ends with the aircraft stable on the centerline, not zigzagging down the runway. Watch as many of thee as you want, watch the rudder and watch the bank angle.

    https://www.youtube.com/watch?v=nehZ9v4YB6o
    Do you realize how close that was from an engine strike?

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  20. #40
    Senior Member Gabriel's Avatar
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    BBobby, just out of curiosity, how well (if at all) would the autoland manage a situation like the one faced by this A380? (you can limit your answer to a 747-400 and 747-8 autoland if you want)

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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